Magnetic resonance gyroscope with spectral control

ABSTRACT

A magnetic resonance gyroscope wherein the pump lamp includes an isotopic mixture of Hg, which controls the optical spectra of the pump beam to eliminate  201  Hg alignment moments and to reduce light-induced frequency shifts. The inclusion of quarter-wavelength plates, reflection polarizers, and adjustable magnetic fields acting on the pump and readout lamps permit fine tuning control over the respective spectra to further enhance the rate bias stability of the gyroscope.

The Government has rights in this invention pursuant to Contract No.N00019-78-C-0532 awarded by the Department of the Navy.

FIELD OF THE INVENTION

The present invention relates to gyroscopes and more particularly to anuclear magnetic resonance gyroscope.

BRIEF DESCRIPTION OF THE PRIOR ART

The prior art includes a number of nuclear magnetic resonance gyroscopeswhich obtain rotational information from the phases of precessingnuclear magnetic moments. Examples of such art include U.S. Pat. Nos.3,778,700 and 4,047,974, both of which are assigned to the presentassignee. A type of magnetic resonance gyroscope for which the presentinvention serves as an improvement is the structure disclosed in thementioned U.S. Pat. No. 3,778,700. In that patent, a gyroscope isdisclosed which comprises nuclear magnetic resonance means, including apair of spin generators, for generating the nuclear magnetic resonantsignals in circuit with means for comparing the phases of the nuclearsignals from each spin generator for providing an output signalproportional to the angle of rotation. Each spin generator comprises amercury absorption cell containing ¹⁹⁹ Hg and ²⁰¹ Hg which is subjectedto a DC magnetic H_(o) field and to an AC H₁ magnetic field in adirection perpendicular to the H_(o) field. Each mercury absorption cellin each spin generator is optically pumped by a circularly polarizedbeam of light at a wavelength having an optical center at 253.7 nm. Thepumping beam for each cell is provided from a randomly polarized beam oflight produced by a common pumping lamp and is separated into its planarpolarized components by a Brewster angle polarizer. The linearlypolarized beams of light are reflected to intersect the absorption cellsin each spin generator and are circularly polarized by properly orientedquarter-wave plates. The readout beams for each spin generator arederived from a second conventional lamp and are separated into twolinearly polarized beams by a second Brewster angle polarizer. The beamsthen respectively intersect the mercury absorption cells. Each readoutbeam passes through the absorption cell wherein its plane ofpolarization is oscillated by the Faraday effect at the Larmorfrequency. The readout beams are converted to amplitude modulated beamsin a linear analyzer which provides a periodic amplitude varying signalto a photodetector circuit. An H₁ field generator is in circuit with thephotodetector for generating the H₁ field for the absorption cell ineach associated spin generator. Readout and control circuit means areprovided for comparing the phases of the two resonance signals producedby the isotopes of mercury of each absorption cell to produce an errorsignal for controlling the H_(o) field generator of one of the cells tomaintain its H_(o) equal to the H_(o) of the other cell, and to providean output signal representing the rotation of the gyroscope about thesensitive axis determined by the direction of the H_(o) fields.

Although this type of gyroscope serves as a basis for the presentinvention and operates with generally satisfactory results, it has beenfound that, unless certain refinements are made to the pump and readoutbeam, ²⁰¹ Hg alignment moments and light-induced-frequency shifts (LIFS)are significant and add to the drift terms at the output of thegyroscope. As a result, the accuracy of the gyroscope may be affected.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

The present invention includes a number of refinements to the pumpingand readout beams which results in control of the beam spectra. Aprimary improvement is the utilization of particular isotopecompositions in the pump lamp. Additional refinements may include theadding of an adjustable Zeeman splitting magnet to the pump lamp whichsplits each pump lamp spectral line into two or more lines. By adding aquarterwavelength plate and a reflection polarizer, the relativeintensities of the Zeeman split lines can be changed. Spectral controlof the pump beam results which reduces or eliminates ²⁰¹ Hg alignmentmoments and reduces the effects of light-induced-frequency shifts,thereby improving the rate bias stability. These additional refinementsin a multiple cell gyroscope also lead to reduced differential intensityresponse to lamp spectral shifts and lamp polarization changes, furtherimproving rate bias stability. Except for the reduction of alignmentmoments, a similar set of improvements holds when the same refinementsare incorporated in the readout beam.

Although the present invention is described in terms of gyros utilizing¹⁹⁹ Hg and ²⁰¹ Hg in a pair of absorption cells, the validity of theinvention is believed to cover any case where one or more spin particleshave spin greater than one half.

In summary, the present invention has as its main feature the control ofspectral features to suppress alignment moments permitting thepreviously discussed advantages simultaneous with the achievement ofLIFS₁₉₉ /LIFS₂₀₁ approaching γ₁₉₉ /γ₂₀₁ and intensity divisionindependent of lamp polarization.

The above-mentioned objects and advantages of the present invention willbe more clearly understood when considered in conjunction with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of the prior art showing the centralcomponents arranged in their proper geometrical relationship.

FIG. 2 is a schematic of the readout beam shown for the prior art ofFIG. 1.

FIG. 3 is a schematic of the optical configuration for the pump beam, asincluded in the present invention.

FIG. 4 is a schematic of the optical configuration for the readout beam,as included in the present invention.

FIG. 5 is a block diagram of the circuit for comparing the phase ofsignals from respective spin generators of the present invention andgenerating a gyro signal therefrom.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the basic configuration of the prior art gyroscopedescribed in the previously mentioned U.S. Pat. No. 3,778,700. Thegyroscope is generally indicated by reference numeral 10 and comprises afirst spin generator designated generally by the reference numeral 11and a second spin generator designated generally by the referencenumeral 12. Each spin generator acts as a basic sensing unit for thegyroscope and serves as an oscillator which effectively simultaneouslyoperates at two frequencies, ω₁ and ω₂. The output frequencies of eachspin generator are influenced by the rate of rotation of the gyroscopeabout the predetermined sensitive axis 22 so that the angle of rotationis added algebraically to the phase of each oscillation from the spingenerator. Each output frequency of each spin generator is proportionalto its magnetic field, H_(o), so that the ratio of the frequencies ineach spin generator remains constant in the absence of rotation.

The phases of the oscillation signals from each spin generator are givenby the following equations:

    φ.sub.11 =∫γ.sub.1 H.sub.01 dt+φ.sub.0

    φ.sub.21 =∫γ.sub.2 H.sub.01 dt-φ.sub.0

    φ.sub.12 =∫γ.sub.1 H.sub.02 dt-φ.sub.0

    φ.sub.22 =∫γ.sub.2 H.sub.02 dt+φ.sub.0  (1)

where γ₁ and γ₂ are the absolute gyromagnetic ratios of the Hg nuclei inthe absorption cell; H₀₁ and H₀₂ are the respective magnetic fieldsproportional to the current applied to the coils which produce thefields; φ₀ is the common angle of rotation of the spin generators aboutthe predetermined sensitive axis; φ₁₁ and φ₂₁ are the phases of theoutput signals from spin generator 11 and φ₁₂ and φ₂₂ are the phases ofthe output signals of spin generator 12; and φ₁₁ and φ₁₂ are the phasesof the signals whose frequency is ω₁, while φ₂₁ and φ₂₂ are the phasesof the signals whose frequency is ω₂.

The angle of rotation is obtained by comparing the phases of pairs ofsignals. Neglecting error terms, if the phase difference in the signalsof one frequency from the two spin generators is maintained equal andopposite in sign to the phase difference between the signals of theother frequency, the phase difference at either frequency is twice theangle of rotation of the gyroscope about the sensitive axis. Thus, if

    (φ.sub.11 -φ.sub.12)+(φ.sub.21 -φ.sub.22)=0(2)

then

    φ.sub.11 -φ.sub.12 =2φ.sub.0 and φ.sub.21 -φ.sub.22 =2φ.sub.0                                             (3)

The condition of equation (2) above may be maintained by developing anerror signal from the sum of the respective phase differences of thecorresponding outputs from the two spin generators. The error signal isused as a differential control signal to control the current through oneor more coils which generates the H_(o) magnetic fields to maintain theerror signal at a null. This forces H₀₁ to equal H₀₂.

The gyroscope 10 includes circuitry shown in FIG. 5 which will bediscussed in greater detail for comparing the phases of the outputsignals from the two spin generators 11 and 12 and for generatingcontrol and output signals as described above. The output signalproduced is proportional to the angle of rotation of the gyroscope 10about the sensitive axis.

The spin generator 11 includes an optically pumped and an opticallymonitored magnetic resonance element which comprises a coil assembly(not shown in FIG. 1) having a mercury absorption cell 14 at its center.Similarly, the spin generator 12 includes a mercury absorption cell 15located at the center of a multiple coil assembly (not shown in FIG. 1).A first field coil (not shown) generates a DC magnetic H_(o) fielddesignated generally by the reference numeral 17, for the spin generator11 while a second field coil (not shown) generates a second DC magneticH_(o) field, designated generally by the reference numeral 18, for thespin generator 12.

For clarity, the orientation of the components in the apparatus shown inFIG. 1 will be related to an arbitrary x, y, z-axis coordinate system toaid in visualizing the spatial relationship of the components and toindicate the polarization of the pumping and readout light beams whichwill be described in detail. The x, y, and z-axes are designatedgenerally by the reference numerals 20, 21 and 22, respectively. Thus,the H_(o) field 17 is in the positive z direction, while the H_(o) field18 is in the negative z direction, so that the field 18 is thusantiparallel to the field 17.

Each of the mercury absorption cells 14 and 15 preferably contains twoodd isotopes of mercury, i.e., ¹⁹⁹ Hg and ²⁰¹ Hg. When the DC H_(o)magnetic field has a strength of about 1.3 gauss, the resonancefrequency of ¹⁹⁹ Hg is approximately 1 kHz and the resonance frequencyof ²⁰¹ Hg is about 369 Hz. When each mercury cell is illuminated bylight in a waveband having a nominal optical center at 253.7 nm, themercury atoms in the cell may absorb light in this region and be excitedfrom the ground state to the first excited level by any light at awavelength which is in resonance with transitions from the ground statemercury atoms in the mercury cell.

The ground state atoms of mercury in each absorption cell possessmagnetic moments due only to their intrinsic nuclear angular momentum orspin properties, since all electronic moments cancel out. When acollection or ensemble of such spins is subjected to the influence of asubstantially homogenous static magnetic field H_(o), the orientationsof the magnetic moments will be quantized or split into a series ofground states or levels having predeterminable energy separations. Inthe absence of very strong magnetic fields or optical pumping, themoments are randomly distributed and produce no net magnetic moment. Amacroscopic magnetic moment may be produced in the mercury vapor by theprocess of optical pumping. Circularly polarized light of precisewavelengths to be absorbed by the mercury atoms adds its angularmomentum to the mercury atoms when it is absorbed. Some of this angularmomentum remains behind when the excited atoms reemit the absorbedelectronic excitation energy. This corresponds to a redistribution ofpopulation among the ground state magnetic quantum levels. For ¹⁹⁹ Hg,there are only two such levels, m_(f) =±1/2, and any asymmetry ofpopulations corresponds only to an orientation moment, with a resultantmacroscopic nuclear magnetic moment. For ²⁰¹ Hg, having a nuclear spinof 3/2, there are four levels, m_(f) =±3/2 and ±1/2. For such atoms, theorientation moment is proportional to 3(n_(+3/2) -n_(-3/2))+(n_(1/2) -n₋1/2), where the n's represent the populations in the respective magneticlevels. Again, the orientation moment is observable as a net magneticmoment. There is also an alignment moment proportional to (n_(3/2)+n_(-3/2))-(n_(1/2) +n_(-1/2)). The alignment moment leads to a varietyof effects in the magnetic resonance gyro, most of them tending toproduce errors in rate.

A pumping lamp 23 provides a beam 24 of randomly polarized absorbablelight which may be resolved into components polarized in a first planedesignated by the numeral 25 and in a second plane designated by thenumeral 26. The light output from the lamp 23 is directed upon aBrewster angle polarizer 27 which also acts as a beam splitter. Thecomponents of the light in the plane 26 are transmitted therethrough andare reflected from a mirror 29 in a direction parallel to the z axis 22.The components of the light in the plane 25 are reflected from theBrewster angle polarizer 27 and are reflected from the mirror 30 in adirection parallel to the z axis 22. The linearly polarized lightreflected from the mirror 29 is circularly polarized by the quarter-waveplate 32 and intersects the mercury absorption cell 14, where itperforms the function of optical pumping. The linearly polarized lightreflected from the mirror 30 is circularly polarized by the quarter-waveplate 33 and intersects the mercury absorption cell 15, producingoptical pumping in this cell.

A readout lamp 35 produces a beam of randomly polarized off-resonancelight which contains components of light polarized in the planedesignated by the reference numeral 37 and light polarized in the planedesignated by the reference numeral 38. The beam from the lamp 35undergoes filtering by a filter cell 36 containing ¹⁹⁹ Hg and ²⁰¹ Hgatoms. The filtered beam then intersects the Brewster angle polarizer 40which transmits the components of light polarized in the plane 38 tointersect the mercury cell 14. Similarly, the components of lightpolarized in the plane 37 in the readout beam are reflected from theBrewster angle polarizer 40 and intersect the mercury absorption cell15.

The geometry shown in FIG. 1 is determined in large part by the Brewsterangle. Preferably, each Brewster angle polarizer is made from stacks ofthin plates of fused silica. When the incident light beam is at theBrewster angle, the reflected light beam is linearly polarized with itselectric vector parallel to the plane of the reflecting surface and thetransmitted beam is partially linearly polarized perpendicularly to thepolarization of the reflected beam.

Each of the mercury absorption cells 14 and 15 is also subjected to anAC H₁ field produced by field coils (not shown). The H₁ fields areperpendicular to the H_(o) fields and the readout beams, as shown inFIG. 2.

The H₁ field applied to cell 14 is produced by the field generator 16 incircuit with the output of the spin generator 11 while the H₁ fieldapplied to cell 15 is produced by the field generator 19 in circuit withthe output of the spin generator 12. Each field generator 16 and 19includes a phasestable amplifier for receiving and amplifying the outputof its respective photodetector, and a field coil oriented with respectto the absorption cell which produces an H₁ field along the axis of thefield coil and perpendicular to the H_(o) field.

The alternating magnetic field H₁ has the effect of applying a torque tothe net magnetic moment of the mercury in the absorption cell, causingit to tilt away from the H_(o) field and to process about the axis ofthe H_(o) field at the frequency of the applied H₁ field. The Larmorprecessional frequency is given by:

    ω=-γH.sub.o                                    (4)

where ω is the Larmor precession frequency, γ is the gyro magneticratio, and H_(o) is the applied DC magnetic field. The negative sign inequation (4) demonstrates that a nucleus with a positive gyromagneticratio will precess in a counterclockwise direction when viewed along adirection parallel to the direction of H_(o), i.e., according to theleft-hand rule with the thumb in the direction of H_(o) and the fingersin the direction of ω.

The precessing magnetic moment will have a component which isperpendicular to the H_(o) field and may be considered to rotate aboutthe axis of the H_(o) field.

The readout beams 38 and 37 pass through respective halfwavelengthplates to the mercury cells 14 and 15, respectively, and the angle ofthe plane of polarization is modulated at the precessional frequency bythe Faraday effect on the readout beam caused by the perpendicular ortransverse magnetic moment component rotating about the H_(o) axis. Themodulation of the angle of the plane of polarization of the readout beam38 is converted to an amplitude modulation by passing the polarizationmodulated beam through the linear analyzer 42 and the amplitudemodulation is detected in the photomultiplier 43. Similarly, the readoutbeam 37 is polarization modulated in the mercury cell 15 and is passedthrough the linear analyzer 45 and is detected in the photodetector 46.The output current from each of the photodetectors is amplified and usedto generate the alternating field H₁.

When all of the conditions of loop closure (such as proper gains and nophase shifts) are met precisely, each of the mercury isotopes in thespin generators 11 and 12 will cause the spin generator to oscillate atits respective Larmor precessional frequency as indicated above.

When a beam of plane polarized light having a direction of propagationparallel to a component of magnetization of a magnetized medium iscaused to pass through the medium, the plane of oscillation of the lightmay be rotated through an angle as a result of the Faraday effect. Whena plane polarized beam 38 of light is caused to pass through the mercurycell 14, it will be affected by a magnetic moment component rotating atthe Larmor frequency about the H_(o) axis and as a result the angularorientation of the plane of polarization of the light will oscillatewith respect to time at the Larmor frequency. Thus, the polarizationangle of the light beams 37 and 38 will be modulated by the cells 15 and14, respectively. The analyzers 45 and 42 convert this polarizationangle modulation to intensity modulation. By properly orienting thedirection of the analyzers 45 and 42, the components of this intensitymodulation at a Larmor frequency can be maximized.

Since two isotopes of mercury are contained within each absorption cell,two such signals are produced by each absorption cell, each having beenmodulated at the characteristic Larmor precessional frequency inaccordance with the gyromagnetic ratio for each isotope. Thus, the netoutput signal is amplitude modulated simultaneously at two frequencieswhich correspond to each of the characteristic frequencies of theisotopes in the mercury cell.

Thus far, the gyroscope operation has been described for a gyroscopewhich is fixed in inertial space.

When the gyroscope rotates about the H_(o) axis, the phase relationshipsare affected in accordance with equation (1) above. That is, therelative phase of the signal at each frequency at the output of eachspin generator after rotation is displaced in phase from the signalwhich would have been received under non-rotation conditions. Thisrelative displacement is thus used to provide an output representativeof the degree of rotation of the gyroscope.

FIG. 3 illustrates an improved optical configuration, embodying thepresent invention, for generating the pump beam. Pump lamp 50 departsfrom the prior art in that the pump lamp includes a different isotope,preferably ¹⁹⁹ Hg in place of the conventional ²⁰⁴ Hg. This may beadjusted by adding a small amount of ²⁰⁴ Hg or ¹⁹⁸ Hg if more light iswanted in the ²⁰¹ Hg(a) or (b) hyperfine regions, respectively. For apreferred embodiment a mixture of 84%±5% ¹⁹⁹ Hg and 16%±5% ²⁰⁴ Hg hasbeen found to reduce substantially alignment moment pumping.Alternately, a ²⁰⁴ Hg filter (not shown) could reduce the amount oflight from the ¹⁹⁹ Hg(a) hyperfine component reaching the ²⁰¹ Hg(a)hyperfine absorption region without affecting the ¹⁹⁹ Hg(b) region. Thepump lamp 50 is positioned in proximity with a SmCo magnet forgenerating an axial DC magnetic field. A housing 54 encloses the magnet52 and the pump beam from lamp 50 experiences Zeeman splitting whereineach pump lamp spectral line is split into two or more lines. Anaperture 56 is formed in the housing 54 from which the beam 58 emerges.A quarter-wavelength plate 60 is positioned between the lamp 50 and aBrewster stack reflective polarizer 66. Each hyperfine line of the lampspectrum is split into circularly polarized components of oppositecircular polarization senses due to the axial magnetic field. Bysuitably orienting the optical axis of quarter-wave retardation plate60, either of these components singly, or any combination of them may beselected as the light linearly polarized in the proper orientation to bereflected by polarizer 66. Thus, by adjusting the strength of theFaraday splitting magnetic field and the orientation of waveplate 60,the pump lamp spectrum may be fine-tuned through the absorption spectrumof the atoms in cells 14 and 15 in order to achieve a balance amongdesired parameters including reduction of alignment moments andreduction of the effects of light induced frequency shifts. Thepolarized reflected beam 68 undergoes a 50%-50% beam splitting at asecond Brewster stack 70 which splits beam 68 into two derivative beams72 and 74. In a preferred embodiment of the invention, the polarizer 66includes five plates while the stack 70 includes four plates. Bydividing the reflective polarizer and splitter functions of components66 and 70, instead of utilizing a single Brewster stack for bothpurposes, equal beam intensities and polarization qualities along beams72 and 74 are realized, despite variations in the polarization of thelight from lamp 50. Respective quarter-wavelength plates 76 and 78 areinserted in beams 72 and 74 prior to impinging upon the cells 14 and 15.

The quarter-wavelength plates 76 and 78 produce circularly polarizedlight for optical pumping in the cells 14 and 15. These plates act in asimilar manner to plates 32 and 33 of the prior art structures of FIG.1.

Referring to FIG. 4, the basic optical configuration for readout isillustrated. A lamp 80 is mounted in proximity with a magnet 81 which isidentical to magnet 52, previously discussed. With readout lamp 80, ²⁰²Hg is employed. The hyperfine spectrum of the light from ²⁰² Hg lamp 80lies outside the region of strong absorption by ¹⁹⁹ Hg and ²⁰¹ Hg. Thereadout beam 82 progresses through a quarter-wavelength plate 84,identical to plate 60 (FIG. 3), to an interference filter 86, whichpreferably has a band pass center of 253.7 nm. The interference filterdefines the portion of the spectrum to be used, thereby eliminatingunmodulated light and thus increasing the signal-to-noise ratio of thegyro output. A filter cell 88 then follows which preferably contains ¹⁹⁹Hg+²⁰¹ Hg and a trace of ²⁰² Hg. This further reduces the amount ofreadout light absorbed by the atoms in cells 14 and 15, thereby furtherreducing the tendency of readout light to produce relaxation of theorientation moments. A Brewster stack reflective polarizer 90, identicalto polarizer 66 previously discussed, reflects the beam 92 to Brewsterstack 94, identical with stack 70, previously discussed. The beam issplit along paths 96 and 98 to respective half-wavelength plates 100 and102 which set the plane of polarization in preferred direction. As inthe case of the pump beam configuration, the controllable axial magneticfield at the lamp and the quarter-wave plate 84 in combination permitthe readout beam spectrum to be fine-tuned over a region on either sideof the nominal position of the ²⁰² Hg hyperfine component. While readoutbeam spectral control does not affect the size of the alignment moment,it is useful in reducing readout beam light-induced-frequency-shifteffects and in improving signal-to-noise. This combination of componentsalso has the beneficial attributes of eliminating unbalance between thebeams due to spontaneous polarization changes which may occur in thereadout lamp 80, and of improving the symmetry of polarization qualitybetween the two readout beam paths 96 and 98. The readout beams thenpenetrate the cells 14 and 15. The beams 104 and 108 passing through theabsorption cells 14 and 15, respectively, impinge upon analyzers 106 and110, respectively. These analyzers may be fabricated from Brewsterstacks of silica plates. As will be observed, the beam 104 splits intobeams 112 and 114 which are read by conventional push-pullphotodetectors 116 and 118. The outputs from the photodetectors drive apush-pull or differential amplifier 128, the output of which goes toelectrical filters shown in FIG. 5. In a similar fashion, the beam 108,from absorption cell 15, is split by analyzer 110 into two derivativebeams 120 and 122. These latter-mentioned beams impinge upon push-pullphotodetectors 124 and 126 which drive the push-pull amplifier 130. Theoutput from the amplifier is connected to the electrical filters shownin FIG. 5.

Turning now to FIG. 5, the schematic diagram shows the phase measurementand control system that provides the gyro output. The two resonancesignals from absorption cell 14 are separated by use of narrow band passfilters 132 and 134. Filter 132 is a 1 kHz band pass filter and filter134 is a 369 Hz band pass filter. These are preferably of the digitalphase-locked-loop type. The ¹⁹⁹ Hg signal is at 1 kHz and the ²⁰¹ Hgsignal is at 369 Hz. After filtering, the output from filters 132 and134 respectively drive inputs 136 and 138 of phase difference detectors140 and 142. The phase difference detectors may be of conventionaldesign, but are preferably of the digital type as disclosed in pendingapplication Ser. No. 144,717 by Lincoln S. Ferris, assigned to thepresent assignee. Similarly, the signal outputs from absorption cell 15are separated by use of narrow band pass filters 144 and 146, whichgenerate signals at the inputs 148 and 150 of the respective phasedifference detectors 140 and 142. The phase difference detector output152 develops Δθ.sub. 199 while the phase difference detector 142develops Δθ₂₀₁. These delta signals are combined in adder 154 to developthe differential magnetic field control signal ΔH_(o) to bring the loopback to a null condition. The output 152 from phase difference detector140 may be directly tapped as the gyro output.

From the foregoing, a nuclear gyroscope has been described whichutilizes a mixture of ¹⁹⁹ Hg and ²⁰⁴ Hg in the pump lamp and otheroptical components in combination as discussed in connection with FIGS.3 and 4 which assist in controlling the spectrum of the pump and readoutbeams, thus resulting in reduced alignment moments, improved beambalances, and reduced light-induced-frequency-shift effects therebyimproving the rate bias stability of the gyro output.

For some applications, gyroscopes of reduced complexity and performanceare desired. For these cases, a single-cell gyroscope may be thepreferred embodiment. For such a gyroscope, the effect of the secondcell on the signal processing may be simulated by conventional digitalprocessing means. The physical arrangement would be described by FIG. 3with components 70, 74, 78, and 15 deleted and by FIG. 4 with components94, 98, 15, 108, 110, 120, 122, 124, 126, and 130 deleted. Such asimplified configuration would have the attributes enumerated aboveexcept for those arising from balancing effects between two cells.

It should be understood that the invention is not limited to the exactdetails of construction shown and described herein for obviousmodifications will occur to persons skilled in the art.

I claim:
 1. In a magnetic resonance gyro having at least two resonancecells, a pump beam generator comprising:a pump lamp including theisotope ¹⁹⁹ Hg therein; reflective means for linearly polarizing lightfrom the lamp and positioned downbeam of the lamp; means for splittingthe polarized beam into two beams of substantially equal intensity fortransmission into the absorption cells; a readout lamp including atleast one Hg isotope therein; means for polarizing the light from thereadout lamp; means transmitting the polarized light through theabsorption cells; and differential push-pull means for detecting thelight readout from the cells.
 2. The structure of claim 1 together witha quarter-wavelength plate located between the said pump lamp and thesaid reflective polarizing means.
 3. In a magnetic resonance gyro havingat least two resonance cells, a pump beam generator comprising:a pumplamp including the isotope ¹⁹⁹ Hg therein; reflective means for linearlypolarizing light from the lamp and positioned downbeam of the lamp;means for splitting the polarized beam into two beams of substantiallyequal intensity for transmission into the absorption cells; means forZeeman splitting the beam from the lamp; a quarter-wavelength platelocated between the said pump lamp and the said reflective polarizingmeans; a readout lamp including at least one Hg isotope therein; meansfor polarizing the light from the readout lamp; means transmitting thepolarized light through the absorption cells; and differential push-pullmeans for detecting the light readout from the cells.
 4. In a magneticresonance gyro having at least two resonance cells, a pump beamgenerator comprising at least two absorption cells:a pump lamp includingthe isotope ¹⁹⁹ Hg therein; reflective means for linearly polarizinglight from the lamp and positioned downbeam of the lamp; means forsplitting the polarized beam into two beams of substantially equalintensity for transmission into the absorption cells; a readout lampincluding at least one Hg isotope therein; means for polarizing thelight from the readout lamp; means transmitting the polarized lightthrough the absorption cells; and means for detecting the light readoutfrom the cells.
 5. The structure of claim 4 together with aquarter-wavelength plate located between the said pump lamp and the saidreflective polarizing means.